Comparative Spectroscopic and Reactivity Studies of Sc3-

Comparative Spectroscopic and Reactivity Studies of Sc3-...
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J. Phys. Chem. C 2007, 111, 11823-11828

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Comparative Spectroscopic and Reactivity Studies of Sc3-xYxN@C80 (x ) 0-3) Ning Chen,† Lou-Zhen Fan,‡ Kai Tan,§ Yue-Qin Wu,‡ Chun-Ying Shu,† Xin Lu,*,§ and Chun-Ru Wang*,† Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Beijing 100080, China, State Key Laboratory of Physical Chemistry of Solid Surface, Center for Theoretical Chemistry, Departmental of Chemistry, Xiamen UniVersity, Xiamen 361005,China, and Department of Chemistry, Beijing Normal UniVersity, Beijing 100000, China. ReceiVed: April 26, 2007; In Final Form: May 24, 2007

A series of endohedral fullerenes Sc3-xYxN@C80 (x ) 0-3) with variable encaged moieties and the same C80 cage were synthesized, isolated, and spectroscopically characterized by the laser desorption time-of-flight mass spectrometry (LD-TOF-MS), differential pulse voltammetry (DPV), Fourier transform infrared (FTIR), visible-near-infrared (vis-NIR) absorption spectrometry, and so forth. It was revealed that Sc3-xYxN@C80 (x ) 0-3) have a similar electronic structure, (Sc3-xYxN)6+@(C80)6-. However, because of the relatively larger van der Waals radius of yttrium to that of scandium which induces the size increase of Sc3-xYxN from x ) 0 to x ) 3, the frontier orbitals of this series of endohedral fullerenes dramatically change. For example, Sc2YN@C80 shows similar electronic property with its left neighbor Sc3N@C80 but is quite different with its right neighbor ScY2N@C80. The cycloaddition reactions of N-enthylazomethine ylide with Sc3-xYxN@C80 (x ) 0-3) were carried out, and the regioselectivity of endohedral fullerenes shows the same trend of variation to that of their electronic properties along with the size of endohedral moiety increasing. It was found that Sc3N@C80 and Sc2YN@C80 produce only the [5,6]-pyrrolidine regioisomers, and a critical change in fullerene regioselectivity occurs from ScY2N@C80 where the [6,6]-pyrrolidine regioisomer appears as a minor regioisomer and finally becomes the major regioisomer in the case of Y3N@C80.

Introduction Endohedral fullerenes, featuring as fullerenes encapsulating one or several atoms, ions, or atomic clusters, are highly interesting because of their novel properties1 and potential applications2 in electronics, optics, biology, medicine, nanomaterials, and so forth. Up-to-date scientists have synthesized, isolated, and spectroscopically characterized hundreds of endohedral fullerenes, with the parent fullerene cages varying from C60 to C100, and the encaged species covering hydrogen,3 nitrogen,4 noble gases,5 alkaline earth metals,6 transition metals,7 lanthanide metals,8 and even atomic clusters such as metal carbides9 and metal nitrides.10 Endohedral fullerenes are usually accompanied with electron transfers from the endohedral moieties to carbon cages;1 that is, the molecular electronic structure can be described as Enm+@C2nm-. Because the entrapment of endohedral moieties within fullerene cages prevents it from an exoteric chemical attack, it is generally believed that the reactivity of endohedral fullerenes is dominated by the fullerene cages, and several early experiments showed a similar reactivity of M@C82 (M ) Y, La, Ce, Pr) that seemingly supported this supposition.11 Recently, however, a comparative study13 for (La2)6+@C806- and (Sc3N)6+@C806- revealed their obviously different reactivity; that is, Sc3N@C80 has much lower thermal reactivity toward disilirane than La2@C80, though both of them share the fullerene * Corresponding authors. Phone and Fax: (+86)10-62652120, e-mail: [email protected] (C.-R.W.). Phone and Fax: (+86)10-62568158, email: [email protected] (X.L.). † Institute of Chemistry. ‡ Xiamen University. § Beijing Normal University.

cage Ih-C806-. Another study of Sc3N@C80 and Y3N@C8014a further revealed different regioselectivities of metallofullerenes with different endohedral moieties: the 1,3-dipolar cycloaddition (1,3-DC) reactions were observed to occur at a [5,6] ring junction of Sc3N@C80 but at a [6,6] ring junction of Y3N@C80. For revealing how the size of encaged moiety of endohedral fullerenes affects their chemical reactions, we recently synthesized a series of endohedral fullerenes, that is, Sc3-xGdxN@C80 (x ) 0-3), and made a systematic study on their reactivities.15 The results show clearly that along with the size of Sc3-xGdxN increasing, the fullerene regioselectivity gradually changes from [5,6]-cycloaddition to [6,6]-cycloaddition. However, this study did not relate the reactivity of Sc3-xGdxN@C80 (x ) 0-3) to their electronic properties, because on the one hand, the low yield of Gd-based fullerenes makes it difficult to characterize their electronic properties and, on the other hand, the super paramagnetic property of Gd-based fullerenes prohibit them from the important NMR characterization of the reacting products. Here, we alternatively synthesized a series of endohedral fullerenes Sc3-xYxN@C80 (x ) 0-3) and made a detailed comparative study on their electronic property and reactivity. Because of the relative large amount of Sc3-xYxN@C80 samples, it is possible to fully characterize the endohedral fullerenes via various spectrometries such as the laser desorption time-of-flight mass spectrometry (LD-TOF-MS), differential pulse voltammetry (DPV), Fourier transform infrared (FTIR), visible-nearinfrared (vis-NIR) absorption spectrometry, and so forth. These studies revealed their electronic properties clearly. The cycloaddition of these endohedral fullerenes were also studied, and

10.1021/jp073229a CCC: $37.00 © 2007 American Chemical Society Published on Web 07/19/2007

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Figure 1. Chromatograms of the isolated Sc3-xYxN@C80 (x ) 0-3) shows different retention times for them in Buckyprep column (toluene eluent at 12 mL/min).

the reaction products were also well characterized by HPLC, LD-TOF MS, and 1H and 13C NMR spectrometry. Furthermore, on the basis of the above experimental investigations, we have also performed ab initio calculations on the electronic structures and reactivity of these endohedral fullerenes. The combined experimental and theoretical studies allow us not only to observe how the electronic properties of Sc3-xYxN@C80 change with the size of the encaged moieties but also to relate the fullerene electronic properties to their reactivity directly. Experimental Section Fullerene soot is produced by arc-burning Sc/Y/graphite composite rods (atomic ratio of Sc:Y:C is ca. 1:1:24) under an atmosphere of He (99%)/N2(1%) mixture at 300 Torr. The soot was collected in ambient condition and Soxhlet-extracted with toluene for 20 h. Soot extracts contain both empty fullerenes such as C60 and C70 and endohedral fullerenes such as Sc2@C2n (2n ) 70-90), Sc3-xYxN@C80 (x ) 0-3). Multistage HPLC technique is employed to isolate and purify the endohedral fullerenes, using two complementary columns: Cosmosil Buckyprep and Cosmosil Buckyprep-M. To ascertain a high purity of the samples, each isolated Sc3-xYxN@C80 (x ) 0-3) is confirmed to have a single-peak HPLC profile (Figure 1) and a single-peak in both negative ionic and positive ionic mass spectra of corresponding samples (Figure S3, Supporting Information). For the fullerene reactions, approximately 2.0 mg of purified (99%) metallofullerenes (Sc3N@C80, Sc2YN@C80, ScY2N@C80, and Y3N@C80) are dissolved in 10 mL of o-dichlorobenzene solutions and heated to 115 °C; then, an excess of N-enthyl glycine (25 fold) and 13C-enriched paraformaldehyde (125 fold) were added to them separately under argon atmosphere. The reaction products are purified by HPLC with a Cosmosil Buckyprep-M column (Φ 20 × 250 mm, toluene eluent, flow rate of 12 mL/min). Results and Discussion Synthesis and Isolation of the Sc3-xYxN@C80 (x ) 0-3). Endohedral fullerenes Sc3-xYxN@C80 (x ) 0-3) are prepared by the Kra¨tschmer-Huffman method and isolated by multistage HPLC technique. Sc3N@C80 were isolated from the soot

Chen et al. extract (Sc/C composite rods arc-discharge) and purified via a two-stage high-performance liquid chromatography (HPLC) process. Buckyprep column separates fullerenes according to their size. In the first stage, the Buckyprep column was used to collect the C88-C90 fraction. The Buckyprep-M column, which separates fullerenes according to their polarity, was employed in the second stage to remove the hollow fullerenes (C88, C90) from Sc3N@C80. Stages 1 and 2 were repeated several times until pure Sc3N@C80 was being collected. Y3N@C80 was separated via a similar process (soot extract from Y/C composite rods arc-discharge) as Sc3N@C80; only the collection fractions are different for them because of the different retention times of Sc3N@C80 and Y3N@C80. Sc2YN@C80 and ScY2N@C80 were isolated from the soot extract (Sc/Y/C composite rods arc-discharge) and purified via a four-stage HPLC process. The first stage of HPLC utilized the Buckyprep column to collect the C88-C90 fraction; the second stage was performed in a Buckyprep-M column to remove the hollow fullerenes. The third stage of HPLC utilized the Buckyprep column again, but a recycling function is employed to separate Sc3N@C80, Sc2YN@C80, ScY2N@C80, and Y3N@C80 into four different fractions. The fourth stage HPLC used the Buckyprep column in recycling mode to purify the isolated Sc2YN@C80 and ScY2N@C80. Figure 1 shows that Sc2YN@C80 and ScY2N@C80 have almost the same retention time with Sc3N@C80 and Y3N@C80 off the Buckyprep column respectively, suggesting that Sc2YN@C80 and Sc3N@C80 have similar size and polarity. Moreover, Sc2YN@C80 and ScY2N@C80 have almost the same retention time on Buckyprep-M and Buckyclutcher columns while their retention time on Buckyprep column is similar but different, indicating that they may have similar polarities. Spectroscopic Study of ScxY3-xN@C80 (x ) 0-3). UVvis-NIR spectra of Sc3-xYxN@C80 (x ) 0-3) are measured and shown in Figure 2a. The strongest absorption for Sc3N@C80, Sc2YN@C80, ScY2N@C80, and Y3N@C80 are 385, 378, 430, and 409 nm, respectively. The electronic absorption spectra of four Sc3-xYxN@C80 (x ) 0-3) endohedral fullerenes are similar to each other. Electronic absorptions of endohedral fullerenes mainly depend on the structure and charge state of the carbon cage. It is known that both Sc3N@C80 and Y3N@C80 share the same C80-Ih fullerene cage and the M3N (M ) Sc, Y) donate six electrons to the C80 cage;16 so, the overall resembling absorption spectra of the Sc3-xYxN@C80 (x ) 0-3) suggest that these fullerenes have a similar electronic structure, that is, (Sc3-xYxN)6+@C806-. Figure 1b shows selected range of the FTIR spectra of the four samples. They have similar fingerprint peaks of C806- at approximately 1200 and 1380 cm-1 which further supports above assignments. In fact, so far, all of the isolated M3N@C80 endohedral fullerenes17 such as Gd3N@C80, Dy3N@C80, Tm3N@C80, ScYErN@C80, and Sc2ErN@C80 were reported to have similar geometric structures as well as electronic structures. On the other hand, in the region of 600800 cm-1, which are assigned as the antisymmetric M-N stretching vibration because of the metal dependence,18 a significant difference was observed for Sc3-xYxN@C80 (x ) 0-3). In particular, while a strong absorption was observed in Sc3N@C80 (599 cm-1) and a small absorption split for Y3N@C80 (726 cm-1, 714 cm-1) as previously reported,18 IR absorption split into two parts for Sc2YN@C80 (676 cm-1, 647 cm-1) and ScY2N@C80(736 cm-1, 699 cm-1). The increasing split hints to an increasing distortion of trigonal geometry. These results are similar to those of previous reported Sc3-xGdxN@C80 (x ) 0-3)12 and Sc3-xErxN@C80 (x ) 0-3).18

Study of Sc3-xYxN@C80 (x ) 0-3)

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Figure 2. Spectroscopic characterizations of Sc3-xYxN@C80(x ) 0-3). (a) UV-vis-NIR spectra, (b) FTIR spectra. 1:Sc3N@C80, 2:Sc2YN@C80, 3:ScY2N@C80, 4:Y3N@C80.

C80-Ih has fourfold degenerate highest occupied molecular orbitals (HOMOs) with only two electrons occupied; so, the C80 moiety in M3N@C80 can receive 6 electrons from the M3N moiety to form a stable closed-shell electronic structure, (Sc3-xYxN)6+@C806-. As a result, the HOMOs of M3N@C80 are mainly localized on the C806-, whereas the lowest unoccupied molecular orbitals (LUMOs) are contributed by both C806- and M3N6+.13 As such, the difference in the electronic properties of M3N@C80 with different M3N mieties, if any, should be dominated by the nature of their LUMOs, that is, different composition of the LUMOs from the orbitals of the C80 and M3N fragments. Indeed, Sc3N@C80 and Y3N@C80 were revealed to have a different reactivity in the regioselectivity of C80 cages;14a therefore, some differences in their molecular LUMOs are expected. A comparative study of Sc3-xYxN@C80 (x ) 0-3) will be helpful to distinguish the differences and make clear where the difference originates. Though the main features of UV-vis-NIR spectra (Figure 2a) of Sc3-xYxN@C80 (x ) 0-3) are similar, if we focus on the optical onsets and nearby absorption peaks that correspond to the electronic transitions involving HOMOs and/or LUMOs, surprisingly then, remarkable differences can be found between them. The HOMO-LUMO transition of ScY2N@C80 has a doublet structure with absorption maxima at 665 and 693 nm. These features are quite similar to those of Y3N@C80 which show HOMO-LUMO transitions with absorption maxima at 664 and 693 nm. However, Sc2YN@C80 exhibits singlet HOMO-LUMO transitions at 723 nm which resembles that of Sc3N@C80 (735 nm). Thus, the four Sc3-xYxN@C80 (x ) 0-3) members can be classified into two groups, in which Sc3N@C80 and Sc2YN@C80 showing close resemblances on both absorption on-set and nearby absorption peaks in the spectra are named as group 1 and ScY2N@C80 and Y3N@C80 are similarly named as group 2 (Figure 3). A critical change occurs at the point from Sc2YN@C80 to ScY2N@C80. This result completely agrees with that of recently reported Sc3-xGdxN@C80 (x ) 0-3).12 On the basis of the spectral onsets, their optical energy gaps were calculated. In spite of the obvious difference between these two groups, their optical energy gaps are quite close, which range from 1.50 eV for Sc3N@C80 to 1.71 eV for Y3N@C80. Since an energy gap value of 1.0 eV marks the borderline between large and low-energy gap fullerenes, these four TNT fullerenes are classified as large energy gap fullerenes. Electrochemistry Study of Sc3-xYxN@C80 (x ) 0-3). To get more details of the electronic structures and chemical properties of these fullerenes, cyclic voltammograms (CVs) and the differential pulse voltammograms (DPV) of Sc3-xYxN@C80 (x ) 0-3) in o-dichlorobenzene were studied, and they all show

Figure 3. On-sets and nearby absorption peaks in UV-vis-NIR absorption spectra of Sc3-xYxN@C80 (x ) 0-3).

Figure 4. DPVs of Sc3-xYxN@C80 (x ) 0-3) in o-dichlorobenzene containing 0.1 M TBAPF6 at a 50 mV/s scan rate, 50 mV pulse, 50 ms pulse width, and 200 ms period.

similar features for the four members of this endohedral fullerene family, suggesting similar electronic structures of this series of endohedral fullerenes. However, both CV and DPV of Sc3-xYxN@C80 (x ) 0-3) are far from identical, reflecting their subtle differences in molecular frontier orbitals. Figure 4 shows the DPV of Sc3-xYxN@C80 (x ) 0-3) in o-dichlorobenzene. Obviously, the DPV patterns of Sc2YN@C80 and Sc3N@C80 are essentially the same while those of ScY2N@C80 and Y3N@C80 are also; the main differences were observed to occur beginning from Sc2YN@C80 to ScY2N@C80. Moreover, the

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Chen et al.

Figure 6. Comparative study of the HPLC (Buckyprep-M, toluene, 12 mL/min) profiles of [5,6]-pyrrolidino-Sc2YN@C80 and [5,6]/[6,6]pyrrolidino-ScY2N@C80.

Figure 5. HPLC (Buckyprep-M, toluene eluent at 12 mL/min) profile of the reacting products in the 1,3-dipolar cycloaddition reaction of N-enthylazomethine ylide, in which pyrrolidinofullerenes [C4H9N]Sc3-xYxN@C80 (x ) 0-3) are the main products; (*) denotes odichlorobenzene peak and (2) denotes bisadducts of Sc3-xYxN@C80 (x ) 0-3) fullerenes.

electrochemical gaps of Sc3-xYxN@C80 (x ) 0-3) result from the potential difference between their first reduction and their first oxidation steps which are 1.91, 1.94, 2.07, and 2.15 V, respectively, consistent with the order of optical energy gaps obtained from UV-vis-NIR absorption spectrometry. Chemical Reactivity Study of Sc3-xYxN@C80 (x ) 0-3). Since Sc3-xYxN@C80 (x ) 0-3) show different electronic properties, they must sometimes display different chemical reactivity too. In fact, different regioselectivities for Sc3N@C80 and Y3N@C80 were just reported.14 Now that group 1 and group 2 metallofullerenes separately show similar in-group electronic properties and obvious differences between groups, we may reasonably presume that Sc3-xYxN@C80 (x ) 0-3) have their regioselectivities changing at the point from Sc2YN@C80 to ScY2N@C80 in reactivity. Therefore, we performed a comparative study on the Sc3-xYxN@C80 (x ) 0-3) chemical reactions by following a standard procedure of the 1,3-dipolar cycloaddition reaction of N-enthylazomethine ylide.14,19-23 In our experiments, first, four metallofullerene solutions (Sc3N@C80, Sc2YN@C80, ScY2N@C80 and Y3N@C80 in odichlorobenzene) were heated to 115 °C; afterward, 13C enriched formaldehyde and an excess of N-ethyl glycine were added to react with the metallofullerenes under the same temperature. The experimental conditions for the four samples were carefully kept identical for comparison. After 15 min reaction time, the samples were cooled and HPLC with a Buckyprep-M column was applied to isolate the reacting products. The resulting HPLC profiles for the four samples were shown in Figure 5. Each fraction of the HPLC profiles was collected and analyzed by MALDI-MS to determine the composition, in which the fractions with retention time at 19.6-23.9 min were assigned as the pyrrolidinofullerene monoadducts (Figure S4, Supporting Information), and other fractions were o-dichlorobenzene, the unreacted Sc3-xYxN@C80, and pyrrolidinofullerene biadducts, respectively. Because the cycloaddition reactions of N-enthylazomethine ylide with Sc3N@C80 and Y3N@C80 under similar conditions

have been studied previously,14,19-24 the monoaddition derivative of Sc3N@C80 was known to be the [5,6]-[C4H9N]-Sc3N@C80 regioisomer,14a and that of Y3N@C80 was the [6,6] regioisomer. As shown in Figure 4, the [5,6]-[C4H9N]-Sc3N@C80 and [6,6][C4H9N]-Y3N@C80 have different retention times at 20.1 and 22.7 min, respectively, and the N-ethyl pyrrolidino-Y3N@C80 has a minor regioisomer appear before [6,6]-[C4H9N]-Y3N@C80 which was assigned as the [5,6]-[C4H9N][email protected] The composition of this fraction was determined by MALDI-MS to be [C4H9N]-Y3N@C80, and after the heat treatment of the sample at 180 °C for 1 h, the peak for this fraction increases largely, indicating that most [6,6]-[C4H9N]-Y3N@C80 was converted to [5,6] regioisomer. It shows that the [6,6] addition is kinetically controlled, but the [5.6] adduct is thermodynamically more stable. This phenomena was also reported by Echegoyen et al.22 Obviously, the retention times of the N-ethyl pyrrolidinofullerenes depend mainly on the [5,6] or [6,6] regioisomers but not on the metallofullerene (ScxY3-xN@C80) composition, and then may be used as a convenient criterion to distinguish different regioisomers of the M3N@C80 analogues. In Figure 5, it was observed that the monoaddition pyrrolidino-Sc3N@C80, -Sc2YN@C80, and -ScY2N@C80 nearly have the same retention times, so the derivatives of Sc2YN@C80 and ScY2N@C80 are also the [5,6] regioisomers similar to the [5,6][C4H9N]-Sc3N@C80. However, the [5,6]-[C4H9N]-ScY2N@C80 peak has a broad tail in the HPLC profile, suggesting the presence of the minor [6,6]-pyrrolidino-ScY2N@C80 regioisomer. We collected the pyrrolidinofullerene fractions of Sc2YN@C80 and ScY2N@C80 with retention times at 21.423.9 min, and we performed the HPLC again. It can be seen that the former sample maintains a single peak while the latter sample shows two distinct peaks (Figure 6) in the HPLC profiles. Obviously, the new peak corresponds to [6,6]-[C4H9N]ScY2N@C80 because of its similar retention time (22.4 min) to [6,6]-[C4H9N]-Y3N@C80. It was estimated that the molar ratio of [6,6] to [5,6] regioisomers of C4H9N-ScY2N@C80 is 1:47 from the HPLC chromatography. The above regioisomer assignment based on the HPLC retention times was further confirmed by NMR spectrometry studies on the purified C4H9N-Sc2YN@C80 and the major isomer of C4H9N-ScY2N@C80. Both of their 13C NMR spectra (Figure 7) exhibit an identical single-peak at 69.95 ppm, which is consistent with the single NMR line (70.09 ppm) of the [5,6]pyrrolidino-Sc3N@C80 derivative but is different from the two NMR line patterns of the [6,6]-pyrrolidino-Y3N@C80 derivative.14a

Study of Sc3-xYxN@C80 (x ) 0-3)

J. Phys. Chem. C, Vol. 111, No. 32, 2007 11827 [5,6] regioisomers of Sc3-xYxN@C80 (x ) 0-3) are always lower than that of the corresponding [6,6] regioisomers, indicating that the [5,6] regioisomers are thermodynamically more stable. This prediction is in line with the previous experimental observation that the addition to the [5,6] site of M3N@C80 (M ) Sc, Y) is thermodynamically favored, but the addition to the [6,6] site is kinetically favored.22a However, the energy difference between the [6,6] and the [5,6] regioisomers decreases remarkably from 11.7 kcal/mol for the Sc3N@C80 case to only 0.5 kcal/mol for the Y3N@C80 case, with a sharp decrease (5.4 kcal/mol) observed between Sc2YN@C80 and ScY2N@C80 derivatives. Thus, according to the Hammond postulate,27 the activation energy required for isomerization from the [6,6] regioisomer to the [5,6] regioisomer22a should increase with the increase of the size of the M3N moiety, and thus, the isomerization becomes much more sluggish for the latter two endofullerenes, as was observed in our experiments.

Figure 7. 13C NMR spectra (150 MHz, top spectra) and 1H NMR spectra (600 MHz, bottom spectra) of 13C-enriched N-ethyl pyrrolidinofullerene derivatives of Sc2YN@C80 and ScY2N@C80.

TABLE 1: PBE/DNP-Predicted Key Geometric Parameters (Bond Length in Å, Angle in Degree) of Sc3-xYxN@C80 (x ) 0-3) and Their Formation Energies (RE, in kcal/mol) and Energy Differences (∆E, in kcal/mol) of the [6,6]- and [5,6]Regioisomers of Pyrrolidinofullerenes Sc3N@C80 Sc-N Y-N Sc-Ca Y-Ca [6,6]-adduct [5,6]-adduct ∆Eb

2.03 2.26

-34.1 -45.8 11.7

Sc2YN@C80 1.98 2.14 2.25 2.39 reaction energy -35.6 -45.5 9.9

ScY2N@C80 1.93 2.10 2.23 2.37 -40.8 -45.3 4.5

Y3N@C80 2.05 2.36 -44.3 -44.8 0.5

Nearest M to C(cage) distance. b ∆E ) Etot(6,6-adduct) - Etot(6,5adduct). a

The 1H NMR spectra of them (Figure 7) agree well with such an assignment. Therefore, our experiments clearly revealed that Sc3N@C80 and Sc2YN@C80 produce only the [5,6]-pyrrolidine regioisomers, and a critical change in fullerene regioselectivity occurs from ScY2N@C80 where the [6,6]-pyrrolidine regioisomer appears as a minor regioisomer and finally becomes the major regioisomer in the case of Y3N@C80. This result agrees well with the electronic property study of Sc3-xYxN@C80 (x ) 0-3). Finally, the size effect of M3N acting on the metallofullerene property and reactivity was explored by means of density functional theory. The geometric structures of Sc3-xYxN@C80 (x ) 0-3) were optimized at the PBE/DNP level of theory with the Dmol3 code.25 As shown in Table 1, the M-N/M-C (M ) Sc, Y) bond lengths decrease continuously from Sc3N@C80 to Y3N@C80, revealing that M3N nuclei must bear increasing stresses along with the increase of the nuclear size. In the meantime, C80 must endure increasing stress and structural distortion due to the increasing size of the M3N moiety. It is natural to imagine that the structural distortion of the fullerene cage would induce a substantial change of the fullerene’s electronic property and reactivity.20 To examine the different reactivities of the four metallofullerenes Sc3-xYxN@C80 (x ) 0-3), the formation energies of [5,6]- and [6,6]-pyrrolidino regioisomers were calculated. For simplicity, we adopted a simplified structural model, in which the -C2H5 group in the pyrrolidine part is replaced by a -H group.26 As shown in Table 1, the calculated energies of the

Conclusion Through comparative studies of Sc3-xYxN@C80 (x ) 0-3), it is observed that Sc3N@C80 and Sc2YN@C80 have nearly the same electronic structures and frontier orbitals, and ScY2N@C80 and Y3N@C80 do as well. A critical transformation on the molecular frontier orbitals occurs at the point from Sc2YN@C80 to ScY2N@C80. The chemical reactivity of Sc3-xYxN@C80 (x ) 0-3) is checked via the 1,3-dipolar cycloaddition reaction of N-enthylazomethine ylide, and the results confirm the critical change of regioselectivity between Sc2YN@C80 and ScY2N@C80. These findings show that the size of the endohedral moiety plays an important role in affecting the electronic properties and chemical properties of endohedral fullerenes, even though they have similar geometric structures. Acknowledgment. The authors thank Professor Echegoyen and Dr. Cardona for in-depth discussions on the fullerene reactivity. C.R.W. thanks NSFC (Nos. 20573132, 90206045, and 20121301), the Major State Basic Research Program of China “Fundamental Investigation on Micro-Nano Sensors and Systems based on BNI Fusion” (Grant 2006CB300402). X.L. thanks NSFC (Nos. 20425312, 20673088, 20423002, 20021002, and 20203013). Supporting Information Available: Details of the separation of ScxY3-xN@C80 (x ) 0-3), Chromatograms and mass spectra of isolated ScxY3-xN@C80 (x ) 0-3), detailed spectroscopic data of ScxY3-xN@C80 (x ) 0-3), mass spectra of monoadduct of pyrrolidinofullerenes,. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843-892. (2) (a) Klingeler, R.; Kann, G.; Wirth, I.; Eisebitt, S.; Bechthold, P. S.; Neeb, M.; Eberhardt, W. J. Chem. Phys. 2001, 115, 7215-7218. (b) Bolskar, R. D.; Benedetto, A. F.; Hudebo, L. O.; Price, R. E.; Jackson, E. F.; Wallace, S.; Wilson, L. J.; Alford, J. M. J. Am. Chem. Soc. 2003, 125, 5471-5478. (c) Krause, M.; Dunsch, L. Angew. Chem. Int. Ed. 2005, 44, 1557-1560. (d) Li, C. J.; Guo, Y. G.; Li, B. S.; Wang, C. R.; Wan, L. J.; Bai, C. L. AdV. Mater. 2005, 17, 71-73. (3) Komatsu, K.; Murata, M.; Murata, Y. Science 2005, 307, 238240. (4) (a) Murphy, T.; Pawlik, T.; Weidinger, A.; Hohne, M.; Alcala, R.; Spaeth, J. M. Phys. ReV. Lett. 1996, 77, 1075-1078. (b) Kanai, M.; Porfyrakis, K.; Briggs, G. A.; Dennis, T. J. S. Chem. Commun. 2004, 210211. (5) (a) Saunders, M.; Cross, R. J.; Jimenez-Vazquez, H. A.; Shimshi, R.; Khong, A. Science 1996, 271, 1693-1697. (b) Syamala, M. S.; Cross, R. J.; Saunders, M. J. Am. Chem. Soc. 2002, 124, 6216-6219.

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